Simultaneous laser frequency stabilization to an optical cavity and an iodine frequency reference

In this Letter, we demonstrate a method to combine a molecular iodine absolute frequency reference with a high-finesse optical cavity in a single laser to take advantage of the frequency stability properties of both systems at different time scales. The result is a laser exhibiting the long-term and short-term stability levels of the iodine frequency reference and optical cavity, respectively. The method uses frequency offset side-band locking and an acousto-optical modulator driven ac-coupled servo-loop to correct the iodine's short-term frequency fluctuations. Experimental results show cavity-limited stability at 1 Hz of 10^-151/Hz and iodine stability below 10 mHz of 10^-131/Hz. In terms of the Allan deviation, this corresponds to stability levels close to the 10^-15 at 1 s and 10^-14 for observation times >100s.